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Toxocariasis

Introduction

Toxocariasis is an infection caused by worms commonly found in the intestines of puppies and older dogs (Toxocara canis), and cats (Toxocara cati). Toxocara worms produce a large number of eggs in the dog or cat’s stomach, and the eggs then leave their body in their faeces, contaminating soil and other surfaces.

Humans become infected by accidentally ingesting (swallowing) worm eggs, or by eating food that is contaminated with soil containing the eggs (such as unwashed raw vegetables). Toxocariasis cannot be spread from person to person. Once the eggs are in the human body they hatch and the worms begin to burrow through tissue.

There are three forms of toxocariasis:

Covert toxocariasis – this form of the disease is by far the most common. It has mild symptoms or may have no symptoms at all.

Visceral larva migrans – this form of the disease develops due to severe or repeated toxocariasis infection. It can cause swelling of the body’s organs or central nervous system (the brain, spinal cord and the nerves connected to it).

Ocular larva migrans – this form of the disease occurs when a Toxocara worm enters an eye. It can cause reduced vision, and in some cases blindness, in the affected eye.

The severity of a toxocariasis infection depends on the number of worm eggs swallowed and the severity of the reaction to them.

Children are most likely to develop toxocariasis, especially children aged 2-7 years. This is because children are more likely to come into contact with soil or sand that contains puppy or cat faeces, and are less likely to follow sensible hand-washing hygiene.

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Symptoms

The symptoms of toxocariasis are due to the body’s allergic reaction to the presence of the Toxocara worm, as well as the worm moving through the organs and tissues of the body. After Toxocara eggs are ingested, it takes at least one week for symptoms to appear, but it may take several weeks or even months. 

Most cases of toxocariasis are of the mild form. Many of these do not have any symptoms at all and are never diagnosed. However, toxocariasis can produce a range of symptoms, including:

  • fever,
  • cough,
  • wheezing,
  • reduced appetite,
  • itchy, swollen red bumps or patches on the skin,
  • difficulty sleeping,
  • pain in the abdomen
  • headaches.

Toxocariasis can also cause enlargement of the liver (hepatomegaly), swollen glands (inflammation of the lymph nodes), and fluid collection in the space surrounding the lungs.

The symptoms of ocular larva migrans can include:

  • reduced vision and eventually blindness,
  • redness of the eye, and
  • whiteness of the pupil (the normally dark centre in the middle of the iris).

Ocular larva migrans can also lead to granulomas (small masses of inflamed tissue) forming on the retina and chorioretinitis - inflammation of the retina and the tissue behind it. Chorioretinitis can cause blurred vision and floating black spots before the eyes.

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Causes

When Toxocara eggs are swallowed they hatch in the intestine. The juvenile (not fully grown) worms burrow through the intestine wall into the bloodstream, which can then carry them to any part of the body.

The juvenile worms can remain alive in the tissues of the body for many weeks, where their movements produce tracks of bleeding. The worms never become fully mature in humans.

Eventually, the worms die. At the point where they die, small abscesses or granulomas (small masses of inflamed tissue) may develop. In some cases, fibrous tissue may build up around the worm, sealing it in.

Direct contact with dogs or cats that have Toxocara worms cannot lead to infection, as Toxocara eggs need to mature in soil for several weeks before they are able to infect humans. Toxocara eggs that are in soil can remain able to infect anyone who comes into contact with them for many months or even years, but will lose the ability to infect if they dry out.

Toxocariasis cannot be spread from person to person.
 
Children who like to put things in their mouths, or those whose families have pet dogs or cats are especially at risk of developing toxocariasis.

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Diagnosis

Toxocariasis is diagnosed by the symptoms and by using an ELISA (enzyme-linked immunosorbent assay) test. An ELISA test can identify Toxocara antibodies in the blood.

Rarely, a biopsy of the liver may be necessary to check for evidence of Toxocara worms or inflammation caused by their presence in the body.

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Treatment

Most people who develop toxocariasis recover without treatment. Many of these will have no symptoms at all and will never realise that they had the infection.

If the disease does produce symptoms, anti-parasitic medicines can be given to try to destroy the juvenile worms. Steroid medicines can also be given to reduce tissue inflammation that develops as a reaction to the Toxocara worm.

Treating ocular toxocariasis can be difficult, but usually involves steroid medicines to prevent or reduce inflammatory reactions to the worm in the eye. In some cases a laser can be used to kill the worm before it reaches the important central area of the retina.

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Prevention

Good hygiene can help prevent toxocariasis:

  • Wash your hands well with soap and water after handling pets or coming into contact with soil or sand.
  • Teach children to always wash their hands after playing with dogs or cats, after playing outdoors, and before eating.
  • Wash food that may have come into contact with soil.
  • Don’t allow children to play in areas that are covered in dog or cat faeces.
  • Teach children that it is dangerous to eat dirt or soil.

Pet owners

Parents and children should be aware of the dangers associated with puppies, kittens and older dogs and cats.

Many puppies are infested with Toxocara worms before birth and both puppies and kittens require de-worming with anti-worm medicine. This should be given at two, three, four and eight weeks after birth, twice more between three and six months of age, and then on one final occasion. Pregnant bitches should be treated with the same medicine. See your vet for specific advice on how to treat your pet.

Clean your pet’s living area at least once a week. Faeces should be either buried or bagged and disposed of in the dustbin.

Toxocara eggs can survive for years in soil or sand, so all dog faeces should be collected and destroyed. Pets should be kept away from children’s sandpits, which should be kept covered when they are not in use.

Some areas within public parks in the UK have been set aside as designated dog exercise areas. Dog-owners should ensure that their dogs use these areas to minimise the risk of toxocariasis for others using the park.

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